I know it is a pain to get smb working with MPlayer CE, and as I have two computers at home,one running XP and the other running Vista. After a day of sitting at the computer I have successfully set up smb with both computers, so I thought I would share how I did it....
With this guide I assume you already have the Homebrew Channel on your wii and MPlayer CE on your SD card. If you are using Vista make sure you are an administrator.
XP:

Spoiler

For Windows XP
1.Insert your SD card into your computer's SD slot.
2. Go to My Computer and open up your SD card's folder.
3. Then go into the folder 'apps'.
4. Then into the folder mplayer_ce.
5. In that folder there should be file called smb or smb.conf(depending on your windows settings).
6. Right click on the file and open it with Notepad or Wordpad (your personal choice).
7.Now minimize that window and click on start.
8.Then click on run and in the box type 'cmd' (without quotation marks) and click OK.
9.A black box should appear with white writing. Now type in 'ipconfig'(without quotation marks) and press enter.
10. A bunch of info should appear. Look at the one that says IPv4 address. After that there should be a bunch of numbers in this format: xxx.xxx.x.x
11. Enter that number ,in smb.conf, next to the bit that says ip1= , It should look something like this ip1=xxx.xxx.x.x but with numbers instead of x.
12. Next to share1 type the name of the folder you want to share... The folder that you want to share must be in the root drive, for example C://Movies. The folder is Movies so my smb.conf should now say: share1=Movies.
13. Enter your username and password next to the user and pass, if you have one.
14. Now click on Start then go to My Computer , in my example,C://Movies, I would open up C and right click on Movies.
15.Then click Sharing and after that tick the box that says 'Share the folder on this network:'
16. Then click OK and save your smb.conf and close up everything. Your done!

VISTA:

Spoiler

For Vista
1.Insert your SD card into your computer's SD slot.
2. Go to Computer and open up your SD card's folder.
3. Then go into the folder 'apps'.
4. Then into the folder mplayer_ce.
5. In that folder there should be file called smb or smb.conf(depending on your windows settings).
6. Right click on the file and open it with Notepad or Wordpad (your personal choice).
7.Now minimize that window and click on start.
8.Then click on All programs, Accessories and then click Command prompt.
9.A black box should appear with white writing. Now type in 'ipconfig'(without quotation marks) and press enter.
10. A bunch of info should appear. Look at the one that says IPv4 address. After that there should be a bunch of numbers in this format: xxx.xxx.x.x
11. Enter that number ,in smb.conf, next to the bit that says ip1= , It should look something like this ip1=xxx.xxx.x.x but with numbers instead of x.
12. Next to share1 type the name of the folder you want to share... The folder that you want to share must be in the root drive, for example C://Movies. The folder is Movies so my smb.conf should now say: share1=Movies.
13. Enter your username and password next to the user and pass, if you have one.
14. Now click on Start then go to My Computer , in my example,C://Movies, I would open up C and right click on Movies.
15.Then click on Properties, then on the tab Sharing then click on Advanced Sharing.
16. If a windows security thing pops click accept.
17. Tick the box that says Share this folder.
18. Then click Permissions, then Add, then Advanced, Find now.
19. A bunch of names should appear at the bottom click the first one that says 'Guest'. Click on it then click OK, which is above the box with names in it.

20. Then click OK, OK, OK, OK and close. But remember to save your smb.conf and close up everything. Your done!

I do not know if the Vista instructions work for Windows 7. Please tell if they do or not. If you have a problem post below.